1. Field of the Invention
The invention relates in general to a bandgap reference circuit, and more particularly to a low voltage bandgap reference circuit.
2. Description of the Related Art
The bandgap reference circuit is widely applied in an integrated circuit, typically for supplying a reference voltage of about 1.25V. The reference voltage is more accurate than a voltage supplied by an external power source and less influenced by temperature and power supply variation. The bandgap reference circuit uses a circuit operating proportional to the absolute temperature to compensate a negative temperature coefficient between a base and an emitter of a bipolar transistor in order to obtain a reference voltage substantially independent of temperature variation.
In order to meet the application requirement of different integrated circuits, a reference voltage smaller than the standard voltage 1.25V is needed. For example, referring to FIG. 1, a circuit diagram of a bandgap reference circuit of a conventional analog system is shown. The circuit derives from a book “DESIGN OF ANALOG CMOS INTEGRATED CIRCUITS” written by Behzad Razavi. In FIG. 1, nodes E and F of a core circuit 110 of a bandgap reference circuit 100 are respectively coupled to two input terminals of an operational amplifier 125 of an additional circuit 120 and resistors are coupled between the two input terminals and two output terminals of the operational amplifier 125. By this design, the bandgap reference circuit 100 can generate a reference voltage, which can be adjusted.
As such, in order to obtain a reference voltage lower than 1.25V, conventionally, an additional circuit, such as the additional circuit 120 of FIG. 1, is employed to be connected to the core circuit of the bandgap reference circuit. The additional circuit is normally composed of complicated analog elements, thereby increasing the circuit area of the whole system and thus the circuit complexity and production cost.